Voltage drops to 10V then back to 14V, mechanic tried everything

My Prius has a huge problem. It started with the triangle and a warning buzzer that wouldnt go off. We replaced the coolant overflow valve and then the pumps. The noise stopped being all the time and then just random. Replaced the battery, the battery terminal since it was getting hot, replaced the inverter pump again. And a few more things. It has been in the shop 5 times and each time Im told it is fixed, I drive it down the road and the buzzer comes back on. I noticed the voltage dropping and everything going dim when this happens, so the last time they hooked up a meter and drove around and each time the buzzer and triangle came on, the voltage dropped down to 10V for 10 or 20 seconds and then back to 14.3 and the buzzer and light would stop. The mechanic thought this was the inverter overheating so he replaced the pump again. I thought it was fixed until the next day going to work it all started again. So I took this video just to document what was happening.

 

The battery also has some kind drain on it when its off. After a couple of days the battery if dead and I have to jump start it, I thought this might be connected but wasnt sure.

 

12V was replaced with new Toyota. Codes are being read by two different machines and a laptop. Im not sure which service he subscribes but Im sure its a good one with all the Prius work he does. I dont remember the codes, but most of them were about the inverter pump.

 

What does WAG mean.

 

Yes its a toyota reader. He works on mostly prius and scion.
I dont know the codes, he rights them down and then pushes some buttons on his laptop and says the codes mean ....something something something and other things I dont understand... and then fixes that. I have never asked for a list of them.

 

Well I have an inverter off a car that we are salvaging due to an accident. Maybe we should just swap them out. It sounds like thats the last thing left to replace. Its got 600,000 miles on it. Thats why I didnt tell him to swap it in the first place.